The present invention concerns a procedure for transforming radioactive wastes into ceramics.
Treatment of the radioactive waste solutions accruing in nuclear energy production aims at transforming the wastes into a safe form for ultimate disposal. In this context, safety implies low solubility of the final waste product, and good mechanical as well as thermal and radiation stability.
In solidifying low and medium-active power plant wastes, the commonest procedures are embedding in concrete and in bitumen. The greatest drawback of the inexpensive and simple embedding in concrete is the high leach rate of radio nuclides from the solidified product. Bituminized products have a lower degree of solubility, but the process is a lot more difficult and risky, e.g., because of risk of ignition. The only procedure widely used in solidifying the high-active waste from reprocessing of spent fuel is vitrifying the waste, particularly in borosilicate glasses. However, experiments carried out with ceramic final waste products such as titanate, zirconate and niobate-based ceramic transformation products have proved that these are superior to glass products in stability, and are gaining ground in research.
Titanates, in particular sodium titanate, are the most important base materials for ceramic products for ultimate disposal. The radioactive wastes are bound to them in the material synthesis, by ion exchange or by mechanical mixing in calcinate form. Thereafter, the product may be transformed into ceramics under high pressure and at high temperature. The most promising ceramic final waste product is SYNROC (A. E. Ringwood et. al., Immobilization of High Level Nuclear Reactor Wastes in Synroc: A Current Appraisal, Research School of Earth Sciences, Australia National University, Publication No. 1975, 1981). It is composed of three minerals, the main components of which are TiO.sub.2 (60%) and ZrO.sub.2 (10%). These minerals are analogous to minerals occurring in nature, and they have been found to have exceedingly low solubility and to tolerate radiation extremely well.
The drawback encumbering the ceramizing procedures studied so far is their complexity and high cost. Expensive initial materials awkward to pre-treat and expensive compressing apparatus are used in them.